Field of the Invention
The present invention relates to a heating device for heating a recording material having an unfixed image formed thereon, to thereby fix the image on the recording material.
Description of the Related Art
Conventionally, a heating device based on an on-demand film heating method has been known as one of heating devices that are each used, e.g. in an electrophotographic copying machine or printer, for thermally fixing an unfixed toner image formed on a recording material, such as a transfer material or a photosensitive sheet, as a permanent fixed image (see Japanese Patent Laid-Open Publication No. 2002-296955).
A ceramic heater is employed for a heating body used in the heating device based on the film-heating method. The heating body comprises a ceramic substrate (e.g. of alumina or aluminum nitride) which is electrically resistant, heat-resistant, and excellent in thermal conductivity, and a primary circuit including a heating resistor (e.g. of silver palladium) which is pattern-shaped on the substrate e.g. by printing or baking and generates heat when electric power is supplied. The heating body is configured to generate heat with electric power supplied to the heating resistor, and its temperature rises quickly because of its generally low heat capacity.
The heat capacity of a heating device using the above-mentioned heating body is small, so that when recording materials having a small size in the longitudinal direction of the heating body continue to be thermally fixed, the temperature of portions of the heating body where the recording materials do not pass becomes higher than the other portions of the same. In this case, the temperature difference causes gloss unevenness in the longitudinal direction of the heating body. To avoid this, it is required to reduce print speed or the temperature difference.
Alternatively, a plurality of heating resistors are provided, and the temperatures of the heating body in the longitudinal direction thereof are detected by a plurality of temperature-detecting elements, whereby power supply to each of the heating resistors is controlled. More specifically, the heating body is formed by a heating resistor group comprising heating resistors that generate a large amount of heat at respective locations where a recording material of any size passes and heating resistors that generate a large amount of heat at respective locations where only a large-sized one of large-sized and small-sized recording materials passes. Then, power supply to the heating resistors is controlled using the temperature-detecting elements, whereby the heating body is configured such that it can be controlled to a predetermined temperature even when a recording material of whichever size is heated.
In the above-mentioned heating body comprising a plurality of heating resistors, a large temperature difference can be instantaneously caused between a heat generation part and a non-heat generation part within the heating body. For example, the temperature difference is caused when a heater control system or a temperature controller runs away, or when thermal runaway (abnormal temperature rise or abnormal heating) of the heating body occurs. Further, when only one heating resistor is continuously energized, a very large amount of electric power is supplied to the heating resistor, which can cause the above-mentioned temperature difference. When the temperature difference occurs, the heating body can be deformed or broken due to thermal stress. Therefore, to avoid the deformation or breakage of the heating body, it is required e.g. to add a safety element capable of very quick response, which makes the heating device complicated in construction, resulting in an increase in manufacturing costs.
To cope with this problem, there has been proposed a technique in which heating resistors are formed on a ceramic substrate such that they are arranged substantially symmetrically with respect to a substantial center position of the substrate in the lateral direction of the same (see Japanese Patent Laid-Open Publication No. 2006-004861). More specifically, the pattern of heating resistor is formed substantially symmetrically with respect to the center position of the substrate in the lateral direction, such that inner heating resistors are disposed closer to the center position, and outer heating resistors are disposed farther from the center position than the inner heating resistors are. This prevents the temperature difference within the heating body from being increased even when the control system of the ceramic heater or the temperature controller runs away or when only one of the heating resistors is continuously energized, whereby thermal stress applied to the substrate is reduced.
In recent years, improvement in the replaceability of life-limited components has been demanded for easy service operation, and this applies to a fixing film used in a heating device based on the on-demand film heating method. Further, it is desired to facilitate work for mounting a fixing film not only for replacement after delivery of a product but also in a manufacturing process before completion of the product.
However, in Japanese Patent Laid-Open Publication No. 2006-004861, a combination of connector contacts (power-supply electrodes) for supplying electric power to the outer heating resistors and connector contacts for supplying electric power to the inner heating resistors are disposed in one end of the substrate, and connector contacts as common electrodes for the inner and outer heating resistors are disposed in the other end of the substrate. Accordingly, connectors for power supply are attached to the respective opposite longitudinal ends of the substrate.
For this reason, in the case of mounting a fixing film in the heating device or removing or replacing a fixing film in the heating device in a state where both the connectors are attached to the opposite ends of the substrate, the fixing film interferes with the connectors attached to the substrate. Therefore, from whichever direction a fixing film is to be mounted to or removed from the heating body, it is required to detach an associated one of the connectors, which degrades workability. What is more, the number of times of connector attachment and detachment for mounting and removal of a fixing film sometimes increases depending on the work process, which leads not only to degraded workability but also to a lowered reliability of electrical conduction between the connectors and the connector contacts.
Further, when contriving the arrangement of the heating resistor groups or paths for routing conductor traces, it is also necessary to pay attention to securing symmetry in the lateral direction of the substrate.